CD16, an immunoglobulin Fc receptor, functions as the primary receptor mediating antibody-directed cellular cytotoxicity (ADCC) on NK cells. Activation of CD16 upon antibody cross-linking results in lysis of target cells. CD16 is also known to play a role in T cell development and function. In humans, certain auto-reactive IgG immune complexes often result in symptoms of inflammatory response and autoimmune diseases, such as lupus and rheumatoid arthritis. Their extracellular ligand binding domain consists of two immunoglobulin modules and their cytoplasmic domain interacts with immuno-tyrosine activation motif (ITAM) containing co-receptors. To date, there has not been a crystal structure available for the family of Fc receptors. As a result, the molecular mechanism governing the activation of Fc receptors remains largely unknown. We have constructed an E. coli expression system to express the extracellular ligand binding domain of the human CD16 molecule. The initial results of in vitro refolding indicate that the soluble receptor can be successfully refolded into a monomeric form. Crystallization experiments have yielded an orthorhombic form of crystal that diffracted to 2.3 angstrom resolution. A new method of heavy atom screening aided by mass spectrometry was developed to achieve a rapid screening of potential heavy atom derivatives. Using this method, we have identified at least two high-quality heavy atom derivatives for the structure solution. The structure determination is in progress using the multiple heavy atom replacement method. Our goal is to form complexes between Fc fragments and this soluble CD16 receptor molecule and to determine the three-dimensional structure of the complexes. In doing so, we hope to map the corresponding interaction regions and to define important residues on the receptor and the Fc domain that are responsible for the receptor activation. - Natural killer cell receptors, crystal structure, CD16, Fc gamma receptor